Macromolecules 2002, 35, 1967-1972
1967
Fluorocarbon-Organosilicon Copolymer Synthesis by Hot Filament Chemical Vapor Deposition Shashi K. Murthy† and Karen K. Gleason*,‡ Departments of Materials Science and Engineering and Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received July 20, 2001; Revised Manuscript Received December 3, 2001
ABSTRACT: Hot-filament chemical vapor deposition has been used to deposit copolymer thin films consisting of fluorocarbon and organosilicon groups from hexafluoropropylene oxide (HFPO) and hexamethylcyclotrisiloxane (D3). This method offers an easy route to the production of such copolymers as the process does not require a solvent and can be performed in a single step. The presence of covalent bonds between the fluorocarbon and organosilicon moieties in the thin films has been confirmed by infrared spectroscopy; X-ray photoelectron spectroscopy (XPS); and solid-state 19F, 13C, and 29Si nuclear magnetic resonance (NMR) spectroscopy. The film structure consists of chains with linear and cyclic siloxane groups and CF2 groups as repeat units. Cross-linking and termination occur mainly via the siloxane units.
Introduction Fluorocarbon and organosilicon thin films produced by chemical vapor deposition have a wide variety of applications, ranging from biocompatible coatings for medical implants1-6 to low-κ dielectrics in integrated circuits.7-13 Fluorocarbon films have been found to be biocompatible and to have low dielectric constants. Organosilicon films with cross-linked siloxane groups offer the advantage of superior thermal stability [relative to linear poly(dimethylsiloxane)].14 Further, they adhere well to silicon substrates by means of covalent bonds formed with the native oxide on the silicon surface.15 A fluorocarbon-organosilicon copolymer film therefore has the potential to incorporate the desirable attributes of each class of material into a single film. Silicone or fluorocarbon homopolymers can be coated onto surfaces by a number of techniques such as spinon coating, casting, or chemical vapor deposition. An important advantage of chemical vapor deposition (CVD) is the ability to create copolymers that are difficult to synthesize by bulk or solution techniques, such as fluorocarbon-organosilicon copolymers. Organosilicon polymers having fluorocarbon pendant groups of the form CH2CH2(CF2)xCF3 as well as polymers having short fluorocarbon segments attached to siloxane chains have been synthesized by solution chemistry techniques.16-19 CVD offers an easier route to the production of these polymers as the process does not involve a solvent and can be performed in a single step. Among the different CVD techniques available, hotfilament CVD (HFCVD, also known as pyrolytic or hotwire CVD) is unique in several respects. First, HFCVD does not require the generation of a plasma, thereby avoiding defects in the growing film produced by UV irradiation and ion bombardment. In addition, films produced by HFCVD have better-defined chemical structures because there are fewer reaction pathways than in the less-selective plasma-enhanced CVD method. Further, HFCVD has been shown to produce films that * Corresponding author. E-mail:
[email protected]. Fax: (617) 2536480. † Department of Materials Science and Engineering. ‡ Department of Chemical Engineering.
have low degrees of cross-linking.20 Co-deposition of fluorocarbon and organosilicon precursors has been performed using plasma-enhanced CVD methods,21-23 but the resulting films had complex structures without clearly defined spectroscopic features. HFCVD has been used to deposit fluorocarbon films that are spectroscopically similar to poly(tetrafluoroethylene) (PTFE),20 as well as organosilicon films that consist of linear and cyclic siloxane repeat units.24 The precursors used in these investigations were hexafluoropropylene oxide (HFPO) and hexamethylcyclotrisiloxane (D3), respectively. The primary propagating unit in the fluorocarbon films was difluorocarbene, generated by the pyrolysis of HFPO. In the case of the organosilicon films, the decomposition of D3 was postulated to occur in two ways: by the breakdown of the rings into dimethylsilanone and by methyl abstraction from the six-membered ring structure, giving rise to the linear and cyclic repeat units, respectively. This paper describes the deposition of fluorocarbonorganosilicon copolymer thin films by HFCVD from HFPO and D3. The copolymer films have well-resolved bonding environments, and extensive spectroscopic characterization confirms the presence of covalent bonds between CF2 groups and siloxane-based polymeric units in the film. Experimental Section Depositions were performed in a custom-built vacuum chamber on to silicon wafer substrates. The pressure within the chamber was controlled by a butterfly valve connected to an MKS type 252 exhaust valve controller. Substrates were placed on a stage maintained at a low temperature (15 ( 5 °C) by the circulation of chilled water through internal coils. Precursor breakdown was achieved by means of a resistively heated 0.038-cm-diameter Nichrome wire (80% nickel, 20% chromium; Omega Engineering). The frame holding the filament wire was equipped with springs to compensate for thermal expansion of the wire upon heating. The distance between the filament wire and the substrate was 1.4 cm. The filament temperature was measured by a 2.2-µm infrared pyrometer. The spectral emissivity was estimated to be 0.85 on the basis of direct-contact thermocouple experiments. The flow of HFPO gas (donated by DuPont) into the chamber was controlled by an MKS model 1295C mass flow controller
10.1021/ma011286a CCC: $22.00 © 2002 American Chemical Society Published on Web 01/29/2002
1968
Murthy and Gleason
Macromolecules, Vol. 35, No. 5, 2002
Figure 1. FTIR spectra of (a) copolymer, (b) silicone, and (c) fluorocarbon films, all deposited by HFCVD under the same conditions. Table 1. Absorption Band Assignments for FTIR Spectra assignment
copolymer (cm-1)
CF2 rocking CF2 wagging Si-C stretching, CH3 rocking in Si-Me2 Si-C stretching, CH3 rocking in Si-Me3 Si-C stretching, CH3 rocking in Si(Me)2(CF2) Si-O-Si asymmetric stretching CF2 symmetric stretching CF2 asymmetric stretching CH3 symmetric bending in Si-Mex CH symmetric stretching in sp3 CH3 CH asymmetric stretching in sp3 CH3
(MFC). The silicone precursor, D3 (Gelest) was vaporized in a stainless steel vessel that was heated to 90 ( 5 °C. The lines leading from the vessel to the vacuum chamber were maintained at 130 ( 5 °C. The flow of vapor from the vessel into the chamber was regulated by a needle valve. Prior to the deposition of copolymer film, the filament wire was preconditioned by being heated at a constant voltage of 86.5 V and under a flow of HFPO into the reactor at 30 sccm for 20 min at a chamber pressure of 1 Torr. Following this treatment, the power to the filament was turned down over a 5-min span, and the chamber was pumped up to atmospheric pressure to facilitate cleaning and placement of a 4-in. wafer on the stage. Depositions were performed at a filament temperature of 620 °C and a chamber pressure of 1 Torr. The precursor flow rates were 20 sccm for HFPO and 28 sccm for D3. The duration of these depositions ranged between 10 and 30 min. The deposition rate, determined by profilometry, was approximately 250 Å/min. Fourier transform infrared (FTIR) spectroscopy was performed on the deposited films using a Nicolet Magna 860 spectrometer in transmission mode. The spectra were baselinecorrected and normalized to a thickness of approximately 7000 Å. X-ray photoelectron spectroscopy (XPS) was carried out on a Kratos Axis Ultra spectrometer using a monochromatized aluminum K R source. Solid-state NMR spectroscopy was performed on a homebuilt spectrometer comprising a 6.338 T Oxford superconducting magnet and a 3.2-mm Chemagnetics magic angle sample spinning (MAS) probe. For this analysis, approximately 14 mg of film was scraped off wafers from nine 30-min depositions and packed into a zirconia rotor of 11 mm3 internal volume. Sample spinning at the magic angle of 54.7° was performed to mitigate spectral broadening due to strong homonuclear dipolar and anisotropic chemical shift effects. The sample spinning speeds were 5, 25, and 10 kHz for 29Si, 19F, and 13C, respectively.
514 610 808 848 899 1043, 1107 1155 1223 1265 2913 2967
literature (cm-1) 516-520 650 805 845 N/A 1050 1160 1220 1260 2900 2960
ref 37, 38 39 40 40 N/A 40 39 39 40 40 40
29Si NMR experiments were performed with proton crosspolarization (CP) and proton decoupling to enhance the signal and resolution from the low-natural-abundance 29Si nuclei. The 1H-29Si CP time was 5 ms (as determined by contact-time experiments performed by Pryce Lewis et al.24), and the 90° pulse width was 1.3 µs. 29Si spectra were also obtained with fluorine cross-polarization and fluorine decoupling. The purpose of this was to determine which silicon atoms were in close proximity (
